1. Section 1 1 Introduction 1 Section 2 2 Drought Hazard Profile 1

11.Section 11 11 Dam and Levee Failure Hazard Profile

11.1Nature of the Hazard

Dam failures can result from any one or a combination of the following causes:

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  Prolonged periods of rainfall and flooding, which cause most failures;
  
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  Inadequate spillway capacity, resulting in excess overtopping flows;
  
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  Internal erosion caused by embankment or foundation leakage or piping;
  
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  Improper maintenance, including failure to remove trees, repair internal seepage problems, replace lost material from the cross section of the dam and abutments, or maintain gates, valves, and other operational components;
  
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  Improper design, including the use of improper construction materials and construction practices;
  
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  Negligent operation, including the failure to remove or open gates or valves during high flow periods;
  
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  Failure of upstream dams on the same waterway;
  
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  Landslides into reservoirs, which cause surges that result in overtopping;
  
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  High winds, which can cause significant wave action and result in substantial erosion; and
  
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  Earthquakes, which typically cause longitudinal cracks at the tops of the embankments, that can weaken entire structures.
  

Levees are flood control barriers constructed of earth, concrete, or other materials. Levee failure involves the overtopping, breach, or collapse of the levee. Levee failure is especially destructive to nearby development during flood and hurricane events. The northern half of Louisiana is protected by levees on the Ouachita River under the authority of the Vicksburg District of the United States Army Corps of Engineers (USACE). Coastal and southern Louisiana is protected by an extensive levee system. The New Orleans District of the USACE is responsible for 30,000 square miles of Louisiana south of Alexandria, including 961 miles of river levees in the Mississippi River and Tributaries Project, 449 miles of river levees in the Atchafalaya Basin, and 340 miles of hurricane-protection levees. Appendix G contains a graphic of hurricane levee districts in the USACE New Orleans District.

11.2Disaster History

While there are no reports of significant dam failures in Louisiana, the National Performance of Dams Program, a database of dam incidents (events that affect the structural and functional integrity of dams, though not necessarily causing failure and not including ordinary maintenance and repair, vandalism, acts of war, recreational accidents, and sabotage) maintained by Stanford University, lists one incident from the fall of 1985. Park managers at the Cotile Lake Dam/Reservoir in Rapides Parish reported seepage due to sand and gravel deposits that displaced concrete slabs. There was no dam failure or controlled breach reported in this incident.

Levees have been overtopped or breached during flood events and non-flood events. A section of levee along the Mississippi River near Marrero, Louisiana, failed in a non-flood-related event. The causes of levee failure included scouring and erosion of sand from the toe of the river bank, which create an over-steepened slope and resultant instability of the upper bank. Severe scour at the toe resulted as the channel bottom deepened through the sandy substratum. Flow failure in the sands then led to loss of the upper bank. Thus, the location of the failure was controlled by the nature of the geologic deposits beneath the levee, combined with progressive deepening of the river channel at that location.

11.3Severity

The amount of water impounded, and the density, type, and value of development downstream determine the potential severity of dam failure. High hazard dams are dams where failure or improper operation will probably cause loss of human life. Significant hazard dams are those where failure or improper operation results in no probable loss of human life but can cause economic loss, environmental damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be located in areas with population and significant infrastructure. Low hazard potential dams are those where failure or improper operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner’s property. In Louisiana, there are 15 high hazard, 63 significant hazard, and 287 low hazard potential dams. As shown in Table 15, only seven dams have emergency action plans (one dam is located in Texas).

Table 15: Dams with Emergency Action Plans by Hazard Potential
County	Dam Name	H	S	L
Bossier	Bayou Bodcau Dam	
Caddo	Caddo Dam		
Caddo	Wallace Lake Dam	
Concordia	Old River	
E Baton Rouge	Pennington Lake Dam	
Rapides	Conference Center Lake Dam			
Sabine-La Newton-Tx	Toledo Bend	
Total		5	1	1

Similarly, the severity of levee breaches depends upon the amount of nearby development. While levees in the New Orleans District are regularly maintained by the USACE to prevent breaches, the very existence of levees can present a danger to metropolitan New Orleans. Levee construction by the USACE has encouraged the city and its industries to drain marshland protected by the levees. As drainage lowered the water table allowing the top layers of muck (wet peat) to dry, consolidate, and subside, New Orleans has sunk below sea level. Some engineers theorize that a large-scale hurricane that hits just east of the city can overtop levees, trap water within their walls, and inundate the city under more than 20 feet of water.

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